The invention relates to an X-ray computed tomography (=CT) apparatus, comprising                an X-ray source for emitting an X-ray beam, in particular a divergent X-ray beam, along a beam axis (z),        a filter element for attenuating the X-ray beam,        a stage for an object to be investigated with the attenuated X-ray beam,        a 2D X-ray detector with a detection area,        and a gantry system capable of rotating either the entirety of the X-ray source, the filter element and the 2D X-ray detector, or the stage for the object with respect to a rotation axis (x) which is perpendicular to the beam axis (z), wherein the filter element has a spatially varying X-ray absorption capability along a cross direction (y) which is perpendicular to both the beam axis (z) and the rotation axis (x).        
Such a CT apparatus is known from US 2012/0002782 A1.
The use of X-rays in imaging is a well-established technology, in particular in medicine and science. The basic principle is that the X-rays produced by an X-ray source are partially attenuated by an object to be investigated, and that the X-rays which have enough energy to pass through the object are detected by a two dimensional (2D) X-ray detector (camera), resulting in a two-dimensional (2D) image. This image is also referred to as a projection image. Different types of X-ray sources are used in combination with different cameras, however, the basic principle remains.
In order to generate a three-dimensional (3D) image of the object, projection images have to be acquired from different angles. This can be done by rotating the object, or by rotating the X-ray source together with the camera around the object. In both cases a dataset of 2D projection images is generated. These projection images are then used to generate a new dataset of cross-sections by a process called back-projection. It is from this second cross-sectional dataset from which the 3D information can be obtained. This method is also referred to as computed tomography (CT).
A particular field of interest in preclinical research is the investigation of small living animals, such as mice or rats. When dealing with live animals, rotating the animal is often not advisable. Therefore special scanners have been developed where the animal is placed on a bed, which, just like in a clinical scanner, moves inside the scanner while both the X-ray source and the camera rotate around the bed acquiring projection images at multiple angles.
Imaging live animals using X-rays always results in a certain radiation dose; without X-rays there would not be an image. An important point to consider when scanning live animals is to reduce the radiation dose in order to minimize or even avoid any biological effects.
In the past, filters have been placed between the X-ray source and the object; the filters were designed to preferentially absorb the low energy X-rays. Typically, these filters were made of sheets of metal (e.g. aluminum) with a well-defined and uniform thickness.
Using such a filter, the low energy X-rays are predominantly attenuated by the filter and no longer by the animal, and the total radiation dose is reduced.
Increasing the thickness of the filter or changing the filter type (e.g. from aluminum to copper) will absorb more X-rays, thereby further reducing the radiation dose. However, if the X-ray intensity becomes too low, or the X-ray energy becomes too high, the image quality will deteriorate. Therefore, optimal combinations of X-ray energies and filters are looked for in order to minimize the total radiation dose while still obtaining an image with enough information allowing image analysis.
US 2012/0002782 A1 proposes a CT apparatus wherein an X-ray source and a 2D X-ray detector are rotated around a bed for a human patient. A compensation filter with a varying thickness, and therefore a varying X-ray absorption capability, is located in front of the X-ray source. The center of the compensation filter, which shadows X-rays intersecting an axis of rotation, exhibits a minimum thickness, and thus a minimum of X-ray absorption. Another CT apparatus is known from WO 2013/182928 A1, wherein a filter of basically triangular shape is introduced into an X-ray beam from the side, such that the X-ray beam is attenuated to the largest degree at its edge, and less at its center.
The latter filters, or rather beam shaping inserts or absorbers, are intended for equalizing intensity of radiation which reaches the detector array. The absorber provides a thin layer of material in the central part of the beam and thicker layers in periphery areas. The shape of such absorbers is based on the assumption that a human or animal body shows more absorption of X-ray radiation in the central area and less in the periphery. The shape of the absorbing layer applied to the primary beam is inverted to the expected local thicknesses of the scanning object, which finally equalizes the signals on the detector array across illuminated area. In this way, the required dynamic range of detectors used in tomographical and microtomographical set-ups can be reduced.
It is the object of present invention to provide a CT apparatus which allows to further reduce the radiation dose for an object to be investigated, while retaining a high image quality at the same time.